The present invention relates to improvements in X-ray apparatus having ionography imaging chambers of the type wherein a dielectric receptor sheet or an analogous insulating charge-receiving medium is placed into an interelectrode gas gap which is defined by two electrodes of the chamber and contains a compressed high Z gas, such as Krypton, Xenon or Freon.
Ionography imaging chambers are used in apparatus for making X-ray images without resorting to X-ray film. During imaging of an object, the interelectrode gap is filled with high Z gas. The electrodes which flank the receptor sheet in the interelectrode gap are connected with an energy source whereby the compressed gas absorbs X-rays to effect the generation of a charge by a quantum process, such as the photoelectric or Compton effect. The charge results in development of a latent electrostatic image at one side of the receptor sheet, and such latent image is made visible by an electrostatic technique including the deposition of toner particles. The chamber has a preferably slot-shaped aperture for introduction and withdrawal of receptor sheets.
In order to achieve a satisfactory yield, as well as to reduce the exposure of patients to X-rays, the high Z gas in the interelectrode gap is maintained at a superatmospheric pressure, normally at a pressure of 6-20 atmospheres. The pressure of high Z gas is reduced to atmospheric during introduction of a fresh receptor sheet into as well as during withdrawal of a receptor sheet from the interelectrode gap. To this end, the gap is connected with a reservoir and the gas is evacuated therefrom by resorting to a suitable pump or the like. However, even though the pressure of high Z gas in the interelectrode gap equals or closely approximates the pressure of surrounding air whenever a receptor sheet is advanced into or from the imaging chamber, the receptor sheets invariably entrain high Z gas from the imaging chamber during withdrawal of a sheet which carries a latent electrostatic image, and fresh receptor sheets invariably entrain some atmospheric air into the gap during introduction of such sheets into the imaging chamber. This entails losses in expensive high Z gas during withdrawal of sheets from the interelectrode gap and contamination of high Z gas in the gap during introduction of fresh receptor sheets. The likelihood of escape of relatively large quantities of high Z gas during withdrawal of receptor sheets and of admission of relatively large quantities of air into the imaging chamber during introduction of fresh receptor sheets is quite pronounced because the width of the aperture normally exceeds the thickness of a receptor sheet. This cannot be avoided because at least that side of a freshly exposed sheet which carries the latent image should not contact the adjacent surface or surfaces of the imaging chamber during withdrawal of such sheet from the interelectrode gap. Such dimensioning of the aperture insures that the latent image remains at least substantially intact during evacuation of the sheet from the gap.
In order to salvage at least those quantities of high Z gas which remain in the interelectrode gap but are contaminated by air, contaminated gas is withdrawn from the imaging chamber, separated from air, and reintroduced into the interelectrode gap. Such procedure is time-consuming and expensive because the separation of air from a noble gas normally involves liquefaction of air.